In a next-generation wireless relay system for which high-speed transmission of 100 Mbps to 1 Gbps is required, it is assumed that high frequency bands are allocated. However, typically, compared with a low-frequency band signal, a high-frequency band signal has a strong rectilinear propagation characteristic but it is noted that many dead spots occur where radio wave is hard to reach. Therefore, if it is assumed that the transmission power of a base station is the same as that of a wireless relay system currently commercialized, the coverage (service area) of a cell decreases owing to the allocation of a high-frequency band. This situation is not desirable, both with respect to increasing costs due to increase in the number of base stations and the occurrence of frequent handover.
Consequently, a wireless relay system that includes a relay station (relay device) for relaying wireless communication between a base station and a mobile station has been proposed. Typically, since the relay station is available at lower cost than the base station, a whole system may be realized at low cost while a sufficient coverage is secured by introducing the relay station.
As a relay station, a so-called amplify and forward (AF) type and a so-called decode and forward (DF) type have been known. For example, the AF-type relay station amplifies, without modification, a reception signal from a base station and transmits the reception signal to a mobile station. For example, after the DF-type relay station demodulates and decodes a reception signal from a base station once, the DF-type relay station encodes and modulates the signal, converts the signal into a frame format, and amplifies and transmits the signal to a mobile station. According to the DF type relay station, compared with the AF-type relay station, in a reception device (base station or mobile station) that is a relay destination, a favorable reception error rate characteristic may be obtained.
In order to allow data to be retransmitted from the relay station to the communication device (reception device) that is a relay destination, the relay station stores data, transmitted by the relay station, in a retransmission buffer until, for example, the relay station receives a signal (ACK) that indicates reception completion from the reception device. When a signal (NACK/DTX) that indicates reception noncompletion is fed back from the reception device, the relay station retransmits the data to the reception device. Therefore, for example, if it is assumed that wireless communication is relayed from the base station to the mobile station, the relay station stores a large amount of transmission data in the retransmission buffer in a case in which the number of mobile stations (reception devices) that are relay destinations are large. Therefore, buffer overflow may occur in the retransmission buffer.
Regarding a control operation for a buffer memory that corresponds to the retransmission buffer, a technique has been known in which data in the buffer memory and data transmission are controlled in response to the usage state of the buffer memory (for example, refer to Japanese Laid-open Patent Publication No. 2003-319458). In the technique, when a data accumulation amount in a buffer memory in a wireless base station is more than or equal to a predetermined level, a communication control device controls the buffer memory so as to release (discard) data in the buffer memory in the wireless base station. In addition, in the technique, when the usage state of the buffer memory is comparatively inactive, the communication control station performs a control operation for restricting a transmission amount of downlink data to the wireless base station.
By the way, in accordance with the spread of relay stations from now, it is requested that the capacity of the retransmission buffer be reduced so as to downsize the relay station. However, in the technique of the related art described above, a control operation is simply performed based on whether or not the data accumulation amount in the buffer memory is more than or equal to the predetermined level, and it is not intended that the capacity of buffer itself is reduced.